Explicit map based 2D-to-2D localization typically represents the scene by a database of geo-tagged images (references) [1]}, [2]}, [3]}. Figure REF  (a) illustrates the two stages of this localization with 2D references: image retrieval determines the most relevant part of a scene represented by reference images to the visual queries; pose regression obtains the relative pose of query image with respect to the reference images.
